The present invention relates to a method and a device for stabilizing a road vehicle, particularly a passenger car, having a trailer pulled by the road vehicle. In the case of road vehicles with trailers, the road vehicle/trailer combination can begin to snake due to excessive speed, bad road conditions, crosswinds or the like. It may be that, for example, the article xe2x80x9cFDRxe2x80x94die Fahrdynamikreglung von Boschxe2x80x9d, [VDCxe2x80x94The Vehicle Dynamics Control of Bosch] by A. van Zanten, R. Erhardt and G. Pfaff, ATZ Automobiltechnische Zeitschrift 96 (1994) 11, pages 674 through 689 and the SAE paper 973184 xe2x80x9cVehicle Dynamics Controller for Commercial Vehiclesxe2x80x9d by F. Hecker, S. Hummel, O. Jundt, K. D. Leimbach, I. Faye, H. Schramm describe very successful design approaches for stabilizing the dynamics control of vehicles and semitrailer trucks, yet special difficulties arise when pulling trailers, particularly trailers which do not have their own actuators and sensors for dynamics control stabilization. This is true in particular for trailers which are heavy compared to the towing vehicle. Thus, for example, special stability problems occur in the case of passenger cars which are towing travel trailers.
If a snaking movement or pendulum motions occur in a vehicle combination composed of a motor vehicle and trailer, then the trailer oscillates about its vertical axis and, by way of the trailer hitch, also prompts the towing vehicle to oscillate. If the vehicular speed is below a so-called critical speed, then the oscillations are damped. If the vehicular speed is equal to a critical speed, then oscillations are undamped; if the vehicular speed is above a critical speed, then the oscillations increase in amplitude. The value of the critical speed is a function, inter alia, of geometric data such as wheelbase and drawbar length, of the mass and the moment of yaw inertia of the vehicle and of the trailer, and of the slip-angle rigidity of the axles. For vehicle combinations in the passenger car sector, this value typically varies in the range from 90 to 130 km/h. The frequency of the snaking movement or of the pendulum motion is approximately 0.5 to 1.5 Hz.
Accordingly, the object of the present invention is to specify a method and a device by which the directional stability for road vehicles which are pulling a trailer is improved. It is particularly desirable that the design approach according to the invention gets along with low expenditure for sensors.
This objective is achieved by a method according to claim 1 and a device according to claim 12. In this context, to stabilize a road vehicle, particularly a passenger car, having a trailer pulled by the road vehicle, the road vehicle is monitored with respect to snaking movements and, upon detection of a snaking movement, an essentially periodic yaw moment, in particular at least two periods long, which essentially is in phase opposition to the snaking movement, is automatically applied to the road vehicle. In this manner, it is possible to reduce snaking of the road vehicle/trailer combination and to stabilize the combination. To be understood by snaking in this context is that the road vehicle which is pulling the trailer is subject to an essentially periodic lateral acceleration as well as an essentially periodic yaw rate. In this case, it is not a strictly periodic pendulum phenomenon (the vehicle combination does not represent an ideal pendulum), but rather, time fluctuations can occur in the period duration of the pendulum motion of the trailer or semitrailer. These fluctuations are evident as well, for example, in the repeating or recurrent or essentially periodic signal generated by a lateral-acceleration sensor. That is to say, this signal has a period duration, changing within small boundaries, which ideally, however, is to be regarded as constant over time. Correspondingly, the applied, essentially periodic yaw moment is also not strictly periodic. The period duration in the applied yaw moment is also changed corresponding to the fluctuations in the periods of the pendulum motion of the vehicle combination.
For example, to detect a snaking movement, the lateral acceleration of the road vehicle can be measured by a lateral-acceleration sensor. The frequency and the amplitude of the signal, determined with the aid of the lateral-acceleration sensor, are evaluated to discern the snaking. The frequency is derived, for example, from the time interval between successive zero crossings. Snaking is present, e.g., when the frequency thus ascertained lies within a predetermined frequency band and when the amplitude is greater than a threshold value. In this connection, in addition to the lateral acceleration, it is advantageous to consider the speed and/or the steering angle of the vehicle in order to differentiate snaking from steering movements of the vehicle. An example for the detection of snaking is set forth in FIG. 9.
It is particularly advantageous in connection with the present invention to utilize a snaking detection in which at least one lateral-motion-dynamics variable such as the lateral acceleration, the yaw rate or the yaw acceleration, as well as the vehicular speed are determined, the snaking movement being determined as a function of the at least one lateral-motion-dynamics variable and the speed. This is advantageously implemented by checking whether the lateral-motion-dynamics variable and the speed, respectively, are greater than threshold values assigned to them. It is particularly advantageous to measure both the lateral acceleration and the yaw rate. To ascertain a snaking movement, it is furthermore of advantage to measure the steering angle, and to take quick steering movements into account when determining the snaking movements. To that end, a high-pass filter is advantageously provided which is used to filter a signal corresponding to the steering angle. If this high-pass-filtered steering signal is greater than a specific threshold value, then it can advantageously be assumed that no snaking movement is present.
In an advantageous refinement of the present invention, the yaw moment is applied by the automatic braking of the road vehicle, different braking forces being applied on both sides of the road vehicle. Thus, the periodic yaw moment is applied in a particularly advantageous manner without the necessity for steering movements. In addition, this refinement makes it possible to implement the present invention particularly conveniently in vehicles equipped with anti-lock braking systems, even when they have no vehicle dynamics control (VDC, ESP).
In a further advantageous development of the present invention, the essentially periodic yaw moment is applied by automatic one-sided braking of the vehicle. Particularly good stabilization of the road vehicle/trailer combination is achieved in this manner.
In another advantageous embodiment of the invention, the road vehicle is monitored for instability, and the yaw moment is only applied when no instability of the road vehicle is detected.
In a further advantageous refinement of the invention, the trailer has an inertia braking system. After and/or in addition to applying the essentially periodic yaw moment, the road vehicle is automatically decelerated for a short duration in such a way that the inertia braking system of the trailer is triggered.
A further advantageous development of the present invention provides that the short-duration automatic deceleration of the road vehicle is shifted by the amount of a fixed phase from the zero crossing of the snaking movement. This particularly takes into account the inertia of the trailer. The braking is carried out shortlyxe2x80x94by the amount of a certain fixed phasexe2x80x94prior to or after a zero crossing.
For the case when the inertia of the trailer is negligible, the short-duration automatic deceleration of the road vehicle can also be carried out in a zero crossing of the snaking movement.
According to another advantageous embodiment of the invention, the short-duration automatic deceleration of the road vehicle is only carried out when the application of the essentially periodic yaw moment has resulted in a reduction of the snaking movement beforehand.
In another advantageous refinement of the invention, the automatic short-duration deceleration for triggering the inertia braking system of the trailer is implemented by reducing the drive torque of an engine propelling the road vehicle.
According to another advantageous development of the present invention, the road vehicle is automatically accelerated for a short duration.
The invention is used particularly advantageously in conjunction with hydraulic braking systems. However, it can also be utilized in electrohydraulic or in pneumatic or electropneumatic, or in electromechanical braking systems.
The invention has, inter alia, the following advantages:
The procedure of the present invention acts on the towing vehicle and is thus independent of the respective trailer. Accordingly, in advantageous embodiment of the invention, no additional sensors or actuators are implemented on the trailer.
The procedure of the present invention can fall back upon the sensors made available by anti-lock braking systems (ABS), traction-control systems (TCS) or vehicle dynamics controls (VDC). Usually no further sensors are necessary.
The snaking frequency can be learned, i.e., the snaking detection adapts itself independently to the respective vehicle.
When using the method and device of the present invention, it is possible to dispense with a mechanical device at the trailer hitch for detecting the snaking excursion.